Noise of strongly-multimode laser diodes used in interferometric systems

A unified model of the phase and partition noise of a multimode semiconductor injection laser is presented in the context of the assumption that all modes radiate the same average optical power. Noise is shown to be associated with two processes: phase fluctuations common to all modes, arising from carrier fluctuations, and partition and phase fluctuations describable in terms of a random walk of the optical field state on the surface of an abstract sphere. Both processes are driven by spontaneous emission. The model is solved analytically; results for the intensity noise level to be expected at the output of an unbalanced interferometer illuminated by a multimode diode laser are derived. These predictions are confirmed experimentally using a laser with nine modes at full width half maximum